I bought the pickaxe though I believe it is free online nowadays; that served as my reference manual.
I was lucky enough to have a work environment that encouraged employees to extend their horizons so my first projects were work related, and thus well-defined. If you dont have anything particular in mind why not just pick a project you recent completed (doesnt matter how large or small) and redo it in ruby. That will separate the need to think about design while learning a new language and it will also allow you to map functionality of this new language to constructs you are familiar with in other languages.
From there is it a matter of interest and exploration.

True happiness is not getting what you want, it's wanting what you've already got.